<p>Since its discovery almost two decades ago, heterochromatin protein 1 (HP1) has emerged as a major player in the transcriptional regulation of both heterochromatic and euchromatic genes as well as the mechanics of chromosome segregation and the functional and structural organization of the interphase nucleus. Recent years have brought the identification of a myriad of HP1-interacting proteins. Each of these is discussed in relationship to its role in heterochromatin assembly and HP1 function. The breadth of functions represented by HP1-interacting proteins testifies to its pivotal role in the daily operations of the nucleus.</p>